1. Field of the Invention
The present invention relates to a cationic electrodeposition coating composition. In particular, it relates to a cationic electrodeposition coating composition which contains a curing agent that has been blocked with a terminal primary OH-containing propylene glycol monoalkyl ether.
2. Description of the Related Art
Blocked isocyanate curing agents are generally used in cationic electrodeposition coatings. The blocked isocyanate curing agents are obtained by reacting a polyisocyanate compound with a blocking agent which is reacted with the isocyanate groups and stable at ambient temperature, but can regenerate free isocyanate groups when heated to a dissociation temperature or higher. The blocking agents contain an active hydrogen and can be suitably selected according to the type of polyisocyanate compound to be employed.
However, the increasing level of awareness of environmental issues of late have been accompanied in developed countries by efforts to regulate the amounts of hazardous atmospheric pollutants (HAPs). Since the blocked isocyanate curing agents release blocking agents into the atmosphere when heated, the blocked isocyanate curing agents also need to be considered as a substance under HAPs as blocked by a substance which is considered as a HAPs. For example, conventionally used cationic electrodeposition coating compositions contain diphenyl methane diisocyanates (MDI) which are blocked with E-caprolactam and butyl cellosolve. Since both of the blocking agents are HAPs substances, there is the concern that their use is banned through enforcement of the environmental regulatory standards.
It is an object of the present invention to provide a cationic electrodeposition coating composition which contains a blocked isocyanate curing agent that has been blocked with a substance not recognised as a HAPs.
The cationic electrodeposition coating composition of the present invention contains an epoxy-modified base resin having a cationic group and a blocked isocyanate curing agent, wherein the blocked isocyanate curing agent is obtained by reacting a polyisocyanate compound with a terminal primary OH-containing propylene glycol monoalkyl ether as a blocking agent, as expressed by the formula RO(CH(CH3)CH2O)nH (where R is an alkyl group having 1 to 8 carbons, which may be branched, and n is 1 to 3). The polyisocyanate compound described above is e.g. diphenyl methane diisocyanate, and R in the formula for the propylene glycol monoalkyl ether is an n-butyl group and n is 1 to 2.
In addition, an article is coated using the cationic electrodeposition coating composition.
The cationic electrodeposition coating composition of the present invention contains an epoxy modified base resin having a cationic group and a blocked isocyanate curing agent.
The blocked isocyanate curing agent contained in the cationic electrodeposition coating composition of the present invention is obtained by reacting a polyisocyanate compound with a terminal primary OH-containing propylene glycol monoalkyl ether as a blocking agent, as expressed by the formula RO(CH(CH3)CH2O)nH (where R is an alkyl group having 1 to 8 carbons, which may be branched, and n is 1 to 3).
Examples of the polyisocyanate compound include alkylene diisocyanate, such as trimethylene diisocyanate, trimethyl hexamethylene diisocyanate, tetramethylene diisocyanate, and hexamethylene diisocyanate; cycloalkylene diisocyanate, such as bis(isocyanatomethyl)cyclohexane, cyclopentane diisocyanate, cyclohexane diisocyanate, and isophorone diisocyanate; aromatic diisocyanate, such as tolylene diisocyanate, phenylene diisocyanate, diphenylmethane diisocyanate, and diphenylether diisocyanate; aromatic/aliphatic diisocyanate, such as xylylene diisocyanate, and diisocyanate diethylbenzene; triisocyanate, such as triphenylmethane triisocyanate, triisocyanate benzene, and triisocyanate toluene; tetraisocyanate, such as diphenyl dimethyl methane tetraisocyanate; polymerized polyisocyanate, such as dimer or trimer of tolylene diisocyanate; and terminal isocyanate-containing compounds which are obtained by reacting the above polyisocyanate compounds with a low molecular active hydrogen-containing organic compound such as ethylene glycol, propylene glycol, diethylene glycol, trimethylol propane, hydrogenated bisphenol A, hexanetriol, glycerine, pentaerythritol, castor oil and triethanolamine; and the like.
On the other hand, the terminal primary OH-containing propylene glycol monoalkyl ether is a compound expressed by RO(CH(CH3)CH2O)nH. In the formula, R is an alkyl group having 1 to 8 carbons, which may be branched. Specific examples of alkyl groups include methyl groups, ethyl groups, n-propyl groups, isopropyl groups, n-butyl groups, isobutyl groups, t-butyl groups, amyl groups, hexyl groups, octyl groups and 2-ethylhexyl groups. The number n is 1 to 3, but does not have to be an integer. A preferable formula for the propylene glycol monoalkyl ether has R as an n-butyl group and n being a number between 1 and 2.
The reaction between the polyisocyanate compound and the terminal primary OH-containing propylene glycol monoalkyl ether can be conducted using a well-known method. For example, the polyisocyanate compound is dissolved in a solvent which does not contain active hydrogen, then adding thereto a terminal primary OH-containing propylene glycol monoalkyl ether in an amount corresponding to the NCO equivalent in the polyisocyanate compound, in the presence of a urethanizing catalyst such as a tin compound, then heating the mixture and causing the reaction to occur. The reaction can be confirmed as having finished when the isocyanate group absorption spectrum has disappeared in an IR absorption spectrum.
The cationic group-containing epoxy modified base resin, which is another component contained in the cationic electrodeposition coating composition of the present invention, is manufactured by opening the epoxy rings in the starting material epoxy resin by bringing about a reaction with a mixture of a primary amine, secondary amine, tertiary amine acid salt or other amine, a sulfide and an acid. The term xe2x80x9ccationic groupxe2x80x9d in the present specification shall refer to a group which is cationic in itself or a group rendered cationic by an addition of an acid. A typical example of the starting raw material resin is a polyphenol polyglycidyl ether epoxy resin formed from a reaction between bisphenol A, bisphenol F, bisphenol S, phenol novolac, cresol novolac or other polycyclic phenol compound and epichlorohydrin. Another example of the starting raw material resin is an oxazolidone ring-containing epoxy resin as taught in Japanese Patent Application Laid-open No. 5-306327. This epoxy resin is obtained by a reaction between a diisocyanate compound or a bisurethane compound obtained by blocking the NCO groups in a diisocyanate compound with methanol, ethanol or other lower alcohol, and epoxy groups.
The epoxy resin which is the starting raw material can be used after employing a bifunctional polyester polyol, polyether polyol, bisphenol or dibasic carboxylic acid for chain extension, prior to the epoxy ring-opening reaction brought about by the amine or sulfide. Similarly, in order to adjust the molecular weight or amine equivalent, or to improve the heat flow property, some epoxy rings of the epoxy resin may be reacted with 2-ethyl hexanol, nonyl phenol, ethylene glycol mono-2-ethyl hexyl ether, propylene glycol mono-2-ethyl hexyl ether or other monohydroxy compound, prior to the epoxy ring-opening reaction.
Examples of amines which can be used when opening the epoxy rings and introducing the amino groups include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine acid salt, and N,N-dimethylethanolamine acid salt or other primary amine, secondary amine or tertiary amine acid salt. A ketimine blocked primary amino group-containing secondary amine such as amino ethyl ethanol amine methyl isobutyl ketimine may also be used. It is necessary for at least an equivalent amount of these amines to be reacted with the epoxy rings in order to open all of the epoxy rings.
Examples of sulfides include diethyl sulfide, dipropyl sulfide, dibutyl sulfide, dihexyl sulfide, diphenyl sulfide, ethyl phenyl sulfide, tetramethylene sulfide, pentamethylene sulfide, thiodiethanol, thiodipropanol, thiodibutanol, 1-(2-hydroxyethylthio)-2-propanol, 1-(2-hydroxyethylthio)-2-butanol, and 1-(2-hydroxyethylthio)-3-butoxy-1-propanol. Examples of acids include formic acid, acetic acid, lactic acid, propionic acid, boric acid, butyric acid, dimethylolpropionic acid, hydrochloric acid, sulphuric acid, phosphoric acid, N-acetylglycine, N-acetyl-xcex2-alanine and others.
If the starting material epoxy resin contains a hydroxyl group, then a self-crosslinkable epoxy modified based resin can be obtained by an addition reaction between the hydroxyl group and an isocyanate which has been half-blocked with the terminal primary OH-containing propylene glycol monoalkyl ether. The half-blocked isocyanate can be obtained by using the propylene glycol monoalkyl ether in an amount which corresponds to half of the NCO equivalent in the polyisocyanate compound in the manufacture of the blocked isocyanate curing agent.
It is preferable that a number average molecular weight of the cationic group-containing epoxy modified base resin is in the range of 600 to 4,000. A number average molecular weight of less than 600 decreases solvent resistance, corrosion resistance and other properties in the resulting coating film. Conversely, a number average molecular weight in excess of 4,000 not only makes the synthesis process difficult owing to the limited control over the resin solution viscosity, but also makes difficult a handling of the resulting resin during such procedures as emulsification dispersion. In addiction, since it has a high viscosity, the flow property during heating and curing would be adversely affected, which leads to markedly worse external appearance of the coating film. It is preferable that an amino value or sulfonium value of the cationic group-containing epoxy modified base resin is 30 to 150, and more preferably 45 to 120. Should the amino or sulfonium value fall below 30, it is more difficult for a stable emulsion to be obtained, while if the values exceed 150, drawbacks arise with Coulomb efficiency, redissolution and other electrodeposition coating-related operational considerations.
In the cationic electrodeposition coating composition of the present invention, it is preferred that a solid content weight ratio of the cationic group-containing epoxy modified base resin/the blocked isocyanate curing agent is 50/50 to 90/10, and more preferably 60/40 to 80/20. If the ratio falls outside these ranges, curing ability may be adversely affected.
The cationic electrodeposition coating composition of the present invention further contains a neutralizing acid in order to disperse the components in an aqueous medium. Examples of the neutralizing acids include formic acid, acetic acid, lactic acid, propionic acid, boric acid, butyric acid, dimethylolpropionic acid, hydrochloric acid, sulphuric acid, phosphoric acid, N-acetylglycine, N-acetyl-xcex2-alanine and others. An amount of acid can vary with the amino group or sulfonium group content in the cationic electrodeposition coating composition, but it is preferable for the amount thereof to be sufficient to allow water dispersion.
The cationic electrodeposition coating composition of the present invention may additionally contain a pigment and a pigment dispersing resin. There is no particular limitation on the pigment, as long as it is a known pigment. Examples of the pigments include coloring pigment, such as titanium dioxide, carbon black and red iron oxide; extender pigment, such as kaolin, talc, aluminum silicate, calcium carbonate, mica, clay, and silica; corrosion resistant pigment, such as zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc phosphite, zinc cyanide, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, calcium molybdate and aluminum phosphomolybdate. A cationic or non-ionic low molecular weight surfactant and modified epoxy resins which generally contain quaternary ammonium groups and/or tertiary sulfonium groups can be used as the pigment dispersing resin.
The pigment dispersing resin and pigment are mixed in a prescribed amount by using a ball mill, sand grinding mill or other known dispersing device until predetermined particle sizes have attained uniformly to obtain a paste in which the pigment has been dispersed. The pigment-dispersed paste can be used as long as the pigment in the cationic electrodeposition coating composition constitutes 0-50 wt % of the solid content.
The cationic electrodeposition coating composition of the present invention can be prepared by adding a neutralizing acid to a mixture of an epoxy modified base resin having a cationic group and a blocked isocyanate curing agent to disperse in an aqueous medium, and then adding a pigment dispersed paste thereto. An additive, such as surfactant, antioxidant, UV absorbing agent, curing accelerator may be added to the system as needed, at the desired stages.
In the present invention, the cationic electrodeposition coating composition is coated on an article. The article can be one that is subjected to electrodeposition. The cationic electrodeposition coating can be performed according to a known method. Typically, the cationic electrodeposition coating composition is diluted with deionized water to a solid content of 5 to 40 wt % and preferably 15 to 25 wt %, to form an electrodeposition bath containing the cationic electrodeposition coating composition having a pH range of 5.5 to 8.5. Electrodeposition can be conducted at a temperature of 20 to 35xc2x0 C. and a voltage of 100 to 450 V.
A thickness of a film produced by electrodeposition coating can preferably be 5 to 40 xcexcm when dried, and more preferably 10 to 30 xcexcm. It is preferable to control conditions for electrodeposition coating so as to obtain the above mentioned thickness range. It is appropriate for the coating film to be baked at 100 to 220xc2x0 C., and preferably at 140 to 200xc2x0 C. for 10 to 30 minutes.
The electrocoated article may be further coated with an intermediate coat or a top coat. The intermediate coat and top coat can be applied by art known methods from paint and coating conditions as used for a surface of automobiles.